The present invention relates to a read-only memory (ROM) device and, more particularly, to a ROM device incorporated in a one-chip microcomputer.
In recent years, a one-chip microcomputer has been successfully developed. In such a one-chip microcomputer, a user program is incorporated in the form of a mask ROM. In this ROM device, programming of the data pattern is fixed during the manufacture thereof and, in addition, the data pattern cannot be changed during operation. Therefore, the data pattern may be determined prior to the manufacture of such a read-only memory device; strictly speaking, the data pattern must be determined prior to the manufacturing step for forming the data pattern on a chip.
The manufacture of such a mask ROM device should be able: to reduce the cell area of the ROM device occupied on a chip as much as possible in order to enhance the integration density thereof; and to reduce the turn-around time, that is, the time-period from the time when a user orders the content of the ROM device to the time when the user obtains the one-chip microcomputer incorporating a desired ROM.
There have been various types of mask ROM devices; however, the following systems are representative:
(a) A system in which the binary data "0" or "1" of a memory cell corresponds to the presence or absence of an enhancement type transistor at each intersection between word lines and bit lines (hereinafter, referred to as an F system);
(b) A system in which the binary data "0" or "1" of a memory cell corresponds to the high or low level of the threshold voltage of an enhancement type transistor at each intersection between the word lines and the bit lines (hereinafter referred to as a Ci-A-system);
(c) A system in which the binary data "0" or "1" of a memory cell corresponds to the enhancement type or depletion type of a transistor at each intersection between the word lines and the bit lines (hereinafter, referred to as a Ci-B system); and
(d) A system in which the binary data "0" or "1" of a memory cell corresponds to the presence or absence of a contact window which connects one of the bit lines to a transistor at each intersection between the word lines and the bit lines (hereinafter referred to as an N system).
The above-mentioned ROM systems have the following characteristics:
______________________________________ System Cell Area Turn-around Time ______________________________________ F system Small Long Ci-A system Small Medium Ci-B system Small Medium N system Large Short ______________________________________
That is, with regard to the characteristic of the cell area, the F, Ci-A and Ci-B systems are advantageous, while, with regard to the characteristic of the turn-around time, the N system is advantageous.